The present invention relates most generally to microsensors for sensing chemical or biological analytes. More particularly, the present invention is related to deflectable microcantilever sensors used to sense the presence of chemical and/or biological analytes
The construction of rugged, cheap, reliable and small chemical microsensors whose output can be expressed in terms of a measurable electrical signal such as DC conductivity is of current interest. It is desired to construct devices that can detect and identify chemical or biological analytes alone or in a complex mixture. Ideally, such sensors should be able to function in either a liquid or vapor environment. Among the systems receiving attention in this regard are carbon-black organic polymer composites which are deposited by spin or drop coating on interdigitated arrays. Inclusion of the carbon-black component into the active sensor material is for the sole purpose of obtaining a measurable DC conductivity through the non-conductive active polymer material. The introduction of analyte material causes polymer swelling and consequent resistance changes of the polymer films. To identify specific vapors from a suite of possible substances and to determine the concentration of that vapor or to carry out similar measurements on multi-component systems requires the construction of arrays of sensing elements. Pattern recognition techniques or principal component analysis of the output of an array of sensors can be used for purposes of analyte identification and quantification.
A number of shortcomings are associated with the use of the carbon-black organic polymer composites. First, it is difficult to reliably reproduce the performance characteristics of a given set of chemiresistor elements due to uncontrollable variations in composite construction. Second, spin coated or drop coated carbon-black polymer composites are inherently metastable in nature and may change or degrade with time. Third, metastable composite systems may not reliably adhere to a substrate surface. Fourth, repeated exposure of the metastable sensor element to analyte vapor may lead to misleading drifts and/or changes in performance characteristics. Fifth, the carbon in a composite material may slowly release analyte material following exposure to analyte and thus have a slow recovery time. Sixth, the interdigitated arrays generally consist of two componentsxe2x80x94a glass substrate and a metallic thin film or wire along with interface regions. Such complicated structures can lead to adhesion problems. Furthermore, carbon-black cannot be used for biological sensing because sensors based on biological molecules and attached to a substrate cannot effectively incorporate a material such as carbon-black.
Another approach for sensing analytes includes the use of vibrating microcantilever structures. Using this technique, a microcantilever is driven into oscillation at one of its resonant frequencies using external circuitry. The microcantilever itself is coated with an active sensing material. Absorption of analyte molecules on the vibrating cantilever changes the frequency or amplitude of the oscillation and this change is sensed by the electronic circuitry. There are, however, several shortcomings associated with the use of vibrating or oscillating microcantilevers. The sensing materials coated on the microcantilevers can easily delaminate during use. Sensors based on this technology require extensive electronic circuitry, both to drive the microcantilevers into oscillation and to sense the change in microcantilever frequency and/or amplitude upon exposure to analyte. Additionally, fabricating arrays consisting of many, close packed vibrating cantilevers is extremely difficult due to differences in cantilever resonant frequencies and the proximity of the cantilevers to one another. Finally, these vibrating or oscillating microcantilever sensing devices are highly subject to external vibration or movement, making fabrication of truly portable devices difficult. The present invention addresses the shortcomings of each of the foregoing microsensor technologies and provides a microsensor which uses a microcantilever and a sensing element formed beneath the microcantilever and in contact with the microcantilever. The sensing material is chosen so that in the presence of the desired analyte material, the sensing element undergoes a volumetric expansion or contraction including in the vertical direction. Such a volumetric change causes the upward or downward deflection of the initially stationary microcantilever. The microcantilever includes at least one measurable physical property which changes when the microcantilever deflects in response to the volumetric change of the subjacent sensing material. The microcantilever need not be driven onto oscillation so the associated extensive electronic circuitry is not required.
The present invention provides a method and apparatus for determining the presence and quantity of biological and/or chemical analytes. A deflectable arm of a microcantilever is disposed over and in contact with a sensing material formed on a surface. The sensing material is chosen to undergo a volumetric expansion or contraction in response to the presence of an analyte or analytes desired to be detected. A volumetric change in a vertical direction of the sensing material causes the deflectable arm of the microcantilever to deflect. The deflecting arm includes at least one measurable physical property which changes when the deflectable arm deflects. This change is measured to determine the presence and quantity of the analyte or analytes of interest.